1. Field of the Invention
The present invention pertains to a method and apparatus for monitoring changes in the intra-thoracic pressure of a patient due to the patient's respiratory activity or cardiac function, and, in particular, to a first technique in which pleural pressure changes due to respiratory effort are monitored based on the changes in pressure in the patient's extra-thoracic arterial circulatory system, and to a second technique in which a patient's vessel distention in the extra-thoracic arterial circulatory system due to respiratory activity or cardiac function are monitored.
2. Description of the Related Art
Numerous patients arrive at a hospital's emergency room each day complaining of a respiratory disorder, such as difficulty breathing, wheezing, shortness of breath, etc. Many of these patient's are incapable of communicating effectively with their physician, for example they may be too young, incapacitated in some way, or have a mental deficiency that prevents effective communication with their caregivers. It would be desirable in such situations if a technique existed for monitoring their respiratory function independent of the patient's ability to communicate, i.e., with regard to the patient's description of the problem. Such a technique would also serve as an objective evaluator of a patient's condition, even if subjective communication were possible.
Conventional methods of assessing respiratory function, including work of breathing, include visually monitoring the respiratory effort of the patient, for example, by observing whether the patient is having difficulty breathing. This provides no objective, measurable indication of the patient's well-being.
A more invasive, yet more objective pulmonary effort measuring technique involves placing an esophageal catheter in the patient's airway and monitoring the pressure within the patient's esophagus. It is also possible to monitor a patient's work of breathing using a mechanical ventilator. However, this requires attaching the patient to the ventilator. These methods are invasive and, therefore, have limited application. For example, when an asthma patient enters the emergency department of a hospital, he or she is usually not on a ventilator, yet work of breathing needs to be assessed and treated immediately. In the ICU, a significant number of patients are at high risk for respiratory failure or have recently been extubated. These patients are not on a ventilator, yet monitoring their work of breathing weighs significantly in the plan of care prescribed for them.
There is also a tremendous need to understand interactions between the heart and lungs of patients in the ICU. For example, any obstructive or restrictive disease, such as chronic obstructive pulmonary disease (COPD) or congestive heart failure (CHF), will result in increased intra-thoracic pressure swings. If the patient's work of breathing is high, blood flow from the heart changes within each breath. To date, a tool does not exist that can illustrate these interactions. Another example occurs when high ventilator pressures are needed. With each ventilator breath, blood flow from the heart changes within each breath. Thus, it is important to determine how low the ventilator pressures need to be to provide adequate ventilation without altering blood flow from the heart. This determination is very difficult to make because the determination will be different for each patient. Without an objective measurement of the hemodynamic effect, this determination cannot be made.
Finally, it is known to monitor the blood pressure of a patient to detect a symptom of a heart disease. For example, it is known to monitor a patient's blood pressure for pulsus paradoxis, which is a greater than normal decrease in systolic pressure and pulse wave amplitude during inspiration. Pulsus paradoxis is associated with circumstances in which respiration is labored and often accompanies such conditions as emphysema, pulmonary embolus, cardiac tamponade, lung cancer, or CHF. Other symptoms of heart disease include:                (1) “waterhammer” pulse, which is associated with aortic insufficiency, and is characterized by a rapid pressure upstroke and rapid fall into diastole;        (2) anacrotic pulse, which is associated with aortic stenosis and characterized by a delayed pulse upstroke;        (3) dicrotic pulse, which is associated with decreased arterial tone and is characterized by an accentuated secondary pulse wave that may feel like heart rate is twice as fast as normal;        (4) pulsus bisferiens, which is associated with combined aortic stenosis and insufficiency and is characterized by double peaks in the pulse waveform; and        (5) pulsus alternans, which is usually associated with heart failure and is characterized by a large pulse wave followed by a small secondary wave.        
Conventional non-invasive blood pressure monitors are only capable of taking a “snap shot” of the patient's blood pressure, i.e., the peak systole and diastole pressure, each time the blood pressure is measured. Thus, they are not suited to detect the dynamic blood pressure changes associated with these blood pressure related symptoms of heart disease.
It is known to monitor the blood pressure continuously, so that blood pressure related symptoms of heart disease, such as pulsus paradoxis, can be readily detected. However, conventional continuous blood pressure monitors are invasive; requiring locating a pressure sensor within the patient's arterial circulatory system.